Network Working Group A. Langley
Internet-Draft W. Chang
Expires: May 5, 2014 Google Inc
Nov 2013
ChaCha20 and Poly1305 based Cipher Suites for TLSdraft-agl-tls-chacha20poly1305-04
Abstract
This memo describes the use of the ChaCha20 cipher with a Poly1305
authenticator in Transport Layer Security (TLS).
Status of this Memo
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This Internet-Draft will expire on May 5, 2014.
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Internet-Draft ChaCha20Poly1305 for TLS Nov 20131. Introduction
Existing TLS [RFC5246] cipher suites either suffer from cryptographic
weaknesses (RC4), major implementation pitfalls (CBC mode block
ciphers) or are difficult to efficiently and securely implement in
software (AES-GCM). In order to improve the state of software TLS
implementations, this memo specifies cipher suites that can be fast
and secure when implemented in software without sacrificing key
agility.
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Internet-Draft ChaCha20Poly1305 for TLS Nov 20133. ChaCha20
ChaCha20 [chacha] is a stream cipher developed by D. J. Bernstein.
It is a refinement of Salsa20 and was used as the core of the SHA-3
finalist, BLAKE.
ChaCha20 maps 16, 32-bit input words to 16, 32-bit output words. By
convention, 8 of the input words consist of a 256-bit key, 4 are
constants and the remaining four are an nonce and block counter. The
output words are converted to bytes and XORed with the plaintext to
produce ciphertext. In order to generate sufficient output bytes to
XOR with the whole plaintext, the block counter is incremented and
ChaCha20 is run again, as many times as needed, for up to 2^70 bytes
of output.
ChaCha20 operates on a state of 16, 32-bit words which are
initialised from the input words. The first four input words are
constants: (0x61707865, 0x3320646e, 0x79622d32, 0x6b206574). Input
words 4 through 11 are taken from the 256-bit key by reading the
bytes in little-endian order, in 4-byte chunks. Input words 12 and
13 are a block counter, with word 12 overflowing into word 13.
Lastly, words 14 and 15 are taken from an 8-byte nonce, again by
reading the bytes in little-endian order, in 4-byte chunks. The
block counter words are initially zero.
ChaCha20 consists of 20 rounds, alternating between "column" rounds
and "diagonal" rounds. Each round applies the following "quarter-
round" function four times, to a different set of words each time.
The quarter-round function updates 4, 32-bit words (a, b, c, d) as
follows, where <<< is a bitwise, left rotation:
a += b; d ^= a; d <<<= 16;
c += d; b ^= c; b <<<= 12;
a += b; d ^= a; d <<<= 8;
c += d; b ^= c; b <<<= 7;
The 16 words are conceptually arranged in a four by four grid with
the first word in the top-left position and the fourth word in the
top-right position. The "column" rounds then apply the quarter-round
function to the four columns, from left to right. The "diagonal"
rounds apply the quarter-round to the top-left, bottom-right
diagonal, followed by the pattern shifted one place to the right, for
three more quarter-rounds.
Specifically, a column round applies the quarter-round function to
the following indexes: (0, 4, 8, 12), (1, 5, 9, 13), (2, 6, 10, 14),
(3, 7, 11, 15). A diagonal round applies it to these indexes: (0, 5,
10, 15), (1, 6, 11, 12), (2, 7, 8, 13), (3, 4, 9, 14).
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Internet-Draft ChaCha20Poly1305 for TLS Nov 2013
After 20 rounds of the above processing, the original 16 input words
are added to the 16 words to form the 16 output words.
The 64 output bytes are generated from the 16 output words by
serialising them in little-endian order and concatenating the
results.
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Internet-Draft ChaCha20Poly1305 for TLS Nov 20134. Poly1305
Poly1305 [poly1305] is a Wegman-Carter, one-time authenticator
designed by D. J. Bernstein. Poly1305 takes a 32-byte, one-time key
and a message and produces a 16-byte tag that authenticates the
message such that an attacker has a negligible chance of producing a
valid tag for an inauthentic message.
The first 16 bytes of the one-time key form an integer, _r_, as
follows: the top four bits of the bytes at indexes 3, 7, 11 and 15
are cleared, the bottom 2 bits of the bytes at indexes 4, 8 and 12
are cleared and the 16 bytes are taken as a little-endian value.
An accumulator is set to zero. For each chunk of 16 bytes from the
input message, a byte with value 1 is appended and the 17 bytes are
treated as a little-endian number. If the last chunk has less than
16 bytes then zero bytes are appended after the 1 byte is appended
until there are 17 bytes. The value is added to the accumulator and
then the accumulator is multiplied by _r_, all mod 2^130 - 5.
Finally the last 16 bytes of the one-time key are treated as a
little-endian number and added to the accumulator, mod 2^128. The
result is serialised as a little-endian number, producing the 16 byte
tag. (The original specification of Poly1305 used AES to generate
the constant term of the polynomial from a counter nonce. For a more
recent treatment that avoids the use of a block cipher in this
fashion, as is done here, see section 9 of the NaCl specification
[naclcrypto].)
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Internet-Draft ChaCha20Poly1305 for TLS Nov 20135. AEAD construction
The ChaCha20 and Poly1305 primitives are built into an AEAD algorithm
[RFC5116], AEAD_CHACHA20_POLY1305, that takes a 32 byte key and 8
byte nonce as follows:
ChaCha20 is run with the given key and nonce and with the two counter
words set to zero. The first 32 bytes of the 64 byte output are
saved to become the one-time key for Poly1305. The remainder of the
output is discarded. The first counter input word is set to one and
the plaintext is encrypted by XORing it with the output of
invocations of the ChaCha20 function as needed, incrementing the
first counter word after each block and overflowing into the second.
(In the case of the TLS, limits on the plaintext size mean that the
first counter word will never overflow in practice.)
The reason for generating the Poly1305 key like this rather than
using key material from the handshake is that handshake key material
is per-session, but for a polynomial MAC, a unique, secret key is
needed per-record.
The Poly1305 key is used to calculate a tag for the following input:
the concatenation of the additional data, the number of bytes of
additional data, the ciphertext and the number of bytes of
ciphertext. Numbers are represented as 8-byte, little-endian values.
The resulting tag is appended to the ciphertext, resulting in the
output of the AEAD operation.
Authenticated decryption is largely the reverse of the encryption
process: generate one block of ChaCha20 keystream and use the first
32 bytes as a Poly1305 key. Feed Poly1305 the additional data and
ciphertext, with the length suffixing as described above. Verify, in
constant time, that the calculated Poly1305 authenticator matches the
final 16 bytes of the input. If not, the input can be rejected
immediately. Otherwise, run ChaCha20, starting with a counter value
of one, to decrypt the ciphertext.
When used in TLS, the "record_iv_length" is zero and the nonce is the
sequence number for the record, as an 8-byte, big-endian number. The
additional data is seq_num + TLSCompressed.type +
TLSCompressed.version + TLSCompressed.length, where "+" denotes
concatenation.
(In DTLS, the sequence number is only 48 bits. Thus, when used in
DTLS, AEAD_CHACHA20_POLY1305 based cipher suites use the
concatenation of the 16-bit epoch with the 48-bit sequence number as
a replacement for TLS's 64-bit sequence number.)
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Internet-Draft ChaCha20Poly1305 for TLS Nov 2013
In accordance with section 4 of RFC 5116 [RFC5116], the constants for
this AEAD algorithm are as follows: K_LEN is 32 bytes, N_MIN and
N_MAX are 8 bytes, P_MAX and A_MAX are 2^64, C_MAX is 2^64+16. An
AEAD_CHACHA20_POLY1305 ciphertext is exactly 16 octets longer than
its corresponding plaintext.
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Internet-Draft ChaCha20Poly1305 for TLS Nov 2013
KEY: 000102030405060708090a0b0c0d0e0f101112131415161718191a1b
1c1d1e1f
NONCE: 0001020304050607
KEYSTREAM: f798a189f195e66982105ffb640bb7757f579da31602fc93ec01ac56
f85ac3c134a4547b733b46413042c9440049176905d3be59ea1c53f1
5916155c2be8241a38008b9a26bc35941e2444177c8ade6689de9526
4986d95889fb60e84629c9bd9a5acb1cc118be563eb9b3a4a472f82e
09a7e778492b562ef7130e88dfe031c79db9d4f7c7a899151b9a4750
32b63fc385245fe054e3dd5a97a5f576fe064025d3ce042c566ab2c5
07b138db853e3d6959660996546cc9c4a6eafdc777c040d70eaf46f7
6dad3979e5c5360c3317166a1c894c94a371876a94df7628fe4eaaf2
ccb27d5aaae0ad7ad0f9d4b6ad3b54098746d4524d38407a6deb3ab7
8fab78c9
The following blocks contain test vectors for Poly1305. The first
line contains a variable length input. The second contains the 256-
bit key and the last contains the resulting, 128-bit tag.
INPUT: 000000000000000000000000000000000000000000000000000000000000
0000
KEY: 746869732069732033322d62797465206b657920666f7220506f6c793133
3035
TAG: 49ec78090e481ec6c26b33b91ccc0307
INPUT: 48656c6c6f20776f726c6421
KEY: 746869732069732033322d62797465206b657920666f7220506f6c793133
3035
TAG: a6f745008f81c916a20dcc74eef2b2f0
The following block contains a test vector for the
AEAD_CHACHA20_POLY1305 algorithm. The first four lines consist of
the standard inputs to an AEAD algorithm and the last line contains
the encrypted and authenticated result.
KEY: 4290bcb154173531f314af57f3be3b5006da371ece272afa1b5dbdd110
0a1007
INPUT: 86d09974840bded2a5ca
NONCE: cd7cf67be39c794a
AD: 87e229d4500845a079c0
OUTPUT: e3e446f7ede9a19b62a4677dabf4e3d24b876bb284753896e1d6
To aid implementations, the next block contains some intermediate
values in the AEAD_CHACHA20_POLY1305 algorithm. The first line
contains the Poly1305 key that is derived and the second contains the
raw bytes that are authenticated by Poly1305.
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Internet-Draft ChaCha20Poly1305 for TLS Nov 20138. Security Considerations
ChaCha20 is designed to provide a 256-bit security level. Poly1305
is designed to ensure that forged messages are rejected with a
probability of 1-(n/2^102) for a 16*n byte message, even after
sending 2^64 legitimate messages.
The AEAD_CHACHA20_POLY1305 algorithm is designed to meet the standard
notions of privacy and authenticity. For formal definitions see
Authenticated Encryption [AE].
These cipher suites require that an nonce never be repeated for the
same key. This is achieved by simply using the TLS sequence number.
Only forward secure cipher suites are defined as it's incongruous to
define a high-security cipher suite without forward security.
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Internet-Draft ChaCha20Poly1305 for TLS Nov 20139. IANA Considerations
IANA is requested to assign the values for the cipher suites defined
in this document from the TLS registry.
IANA is requested to assign a value for AEAD_CHACHA20_POLY1305 in the
registry of AEAD algorithms.
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